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Ask Jeep Service Your Own Question
Jeep Service
Jeep Service, Jeep Owner
Category: Jeep
Satisfied Customers: 4915
Experience:  14 years with AMC/Jeep Specializing with pre-1994 Jeep products.
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I have a 87 yj 2.5 has NO fire coming from the coil to

Customer Question

Hi...I have a 87 yj 2.5 has NO fire coming from the coil to the distubuitor ...ive replapaced the crank sensor. coil ign module and had the ecm repaired and SYILL no fire/wont start im LOST!!
Submitted: 2 years ago.
Category: Jeep
Expert:  Juan Crespo replied 2 years ago.

Hi there. Welcome to our site. Just logged in and saw your question.

Do you still need help? If so, are you working on a Renix (no vacuum advance on distributor) or Motorcraft ignition?

Customer: replied 2 years ago.
Expert:  Juan Crespo replied 2 years ago.

I see. On a Renix system, the signal from the crank position sensor (CPS) goes to the MCU, which then sends it to the ICM; the ICM acts as a relay that fires the coil.

In order for us to know where the problem is, we would have to check the signal as it travels from CPS to MCU to ICM to coil. We would need an oscilloscope to do this. Do you have access to one? - FYI: They're less than $100 online. Click here to see sample.

Customer: replied 2 years ago.
I don't have access to one...I replaced the crank sensor 1st then the icm and the coil... I also pulled out the ECM and sent it off for repair thinking my problem might be there... There WAS a problem with it as the tech said he replaced "diode 27" and said everything else was fine
Customer: replied 2 years ago.
What is an MCU and where is it located? Does this Jeep have a cam sensor or not?
Expert:  Juan Crespo replied 2 years ago.

sorry for the delay...

We were never allowed at the dealer to open-up an MCU, so I wouldn't know what part "diode 27" plays in the spark creation scheme.

MCU is what AMC called the engine control module or ECM. There is no cam sensor in a Renix system; both spark and injector pulse are controlled by the MCU based on engine speed signal. Once the engine is running, MCU will look at MAP and ECT signals to control ignition timing through the ICM.

The only way I know to check these signals is either with a scope, or a high-end multimeter capable of reading frequency (Hz).

Expert:  Juan Crespo replied 2 years ago.

Just occurred to me we could possibly eliminate the CPS signal as being the culprit by checking injector pulse. Do you have a Noid light or LED test light you can use to do that?

Customer: replied 2 years ago.
I have a regular test light
Expert:  Juan Crespo replied 2 years ago.

I will do my best to assist you, but please keep in mind that while I can provide you with technical guidance, working on this type of issue requires you to have not only access to several different automotive tools, but the skills to use them as well.

At this point, I don't think I can offer any other suggestion that could help you solve your issue. I'll opt out of the question and let other experts chime in. Please do not reply until another expert responds or the site will revert the question back to me.

Best Regards.

Expert:  Jeep Service replied 2 years ago.

Hello. Different expert here. In our shop, we have observed this problem many, many times. The problem (according to the Jeep tech service people) is a loss of the crank angle sensor voltage. They recommended that we cut the wire connector out of the harness and off of the end of the crank sensor, then solder and heat shrink the wires back together. They claimed that the wire connector caused enough reduction (we are talking millivolts here) of voltage to not fire the system. I would recommend you try this if you have a soldering gun and the experience to use it. Every time we had one towed in for this, and we cut and soldered the crank sensor, the problem was fixed. Hopefully, this will work for you.

If you have any questions, please feel free to reply.

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